U.S. patent application number 15/043544 was filed with the patent office on 2016-08-25 for communication systems with phase-correlated orthogonally-polarized light-stream generator.
The applicant listed for this patent is GEORGIA TECH RESEARCH CORPORATION. Invention is credited to Gee-Kung Chang, Daniel Guidotti, Mu Xu, Jian Yu Zheng.
Application Number | 20160248515 15/043544 |
Document ID | / |
Family ID | 56693841 |
Filed Date | 2016-08-25 |
United States Patent
Application |
20160248515 |
Kind Code |
A1 |
Zheng; Jian Yu ; et
al. |
August 25, 2016 |
Communication Systems With Phase-Correlated Orthogonally-Polarized
Light-Stream Generator
Abstract
In one aspect, the present disclosure relates to a
communications system which, in one embodiment, includes a
phase-correlated, orthogonally-polarized, light-stream generator
(POLG) for preparing light into phase coherent light streams having
defined states of polarization and spectral composition. In one
embodiment, the POLG includes a light source configured to emit
light having a predetermined wavelength and a polarization
apparatus configured to prepare light from the light source into
particular states of polarization. The POLG also includes a phase
modulator configured to produce light having a plurality of
wavelengths and configured to retard the phase of propagation of
light with a first state of linear polarization while not retarding
the phase of light with a state of linear polarization orthogonal
to the first state of linear polarization when an external electric
field is applied. The POLG also includes an electrical oscillator
configured to periodically apply an electric field to the phase
modulator.
Inventors: |
Zheng; Jian Yu; (Beijing,
CN) ; Chang; Gee-Kung; (Smyrna, GA) ;
Guidotti; Daniel; (Atlanta, GA) ; Xu; Mu;
(Atlanta, GA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
GEORGIA TECH RESEARCH CORPORATION |
Atlanta |
GA |
US |
|
|
Family ID: |
56693841 |
Appl. No.: |
15/043544 |
Filed: |
February 13, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62116069 |
Feb 13, 2015 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04J 14/06 20130101;
H04B 10/532 20130101; H04B 10/548 20130101; H04B 10/25752 20130101;
H04B 10/516 20130101; H04B 10/5051 20130101; H04J 14/02
20130101 |
International
Class: |
H04B 10/548 20060101
H04B010/548; H04B 10/25 20060101 H04B010/25; H04B 10/11 20060101
H04B010/11; H04J 14/06 20060101 H04J014/06; H04J 14/02 20060101
H04J014/02 |
Claims
1. A communications system, comprising: a phase-correlated,
orthogonally-polarized, light-stream generator (POLG) for preparing
light into phase coherent light streams having defined states of
polarization and spectral composition, the POLG comprising: a light
source configured to emit light having a predetermined wavelength;
a polarization apparatus configured to prepare light from the light
source into particular states of polarization; a phase modulator
configured to produce light having a plurality of wavelengths and
configured to retard the phase of propagation of light with a first
state of linear polarization while not retarding the phase of light
with a state of linear polarization orthogonal to the first state
of linear polarization when an external electric field is applied;
and an electrical oscillator configured to periodically apply an
electric field to the phase modulator.
2. The communication system of claim 1, wherein the polarization
apparatus comprises: a first, fixed linear polarizer; a 90 degree
optical retarder; and a rotatable, second linear polarizer.
3. The communication system of claim 1, wherein the phase modulator
comprises a semiconductor crystal having a zinc blende symmetry or
space group symmetry F43m.
4. The communication system of claim 3, wherein the semiconductor
crystal belongs to the class of III-V compound semiconductors.
5. The communication system of claim 1, further comprising a
polarization-selective light flux modulator coupled to the POLG,
the polarization-selective light flux modulator configured to
change the flux of light with a first state of linear polarization
while not changing the flux of light with a state of linear
polarization orthogonal to the first state of linear polarization
when an external electric field is applied.
6. The communications system of claim 5, wherein at least one of
the phase modulator and polarization-selective light flux modulator
comprises a semiconductor crystal including Gallium Arsenide or
Indium Phosphide.
7. The communications system of claim 5, further comprising a data
encoder configured to drive the polarization-selective light flux
modulator and forming, together with the polarization-selective
light flux modulator, a light intensity modulator configured to
encode data on one polarization of light emerging from the
polarization-selective light flux modulator while not encoding data
on light having an orthogonal polarization.
8. The communication system of claim 5, wherein the
polarization-selective light flux modulator is configured as a
Mach-Zehnder interferometer.
9. The communication system of claim 5, wherein the
polarization-selective light flux modulator comprises: a plurality
of optical waveguides extending parallel to a crystal surface
having a perpendicular axis parallel to the z-axis and configured
and oriented such that light propagates in the waveguides parallel
to a y'-direction or a x'-direction while an external electric
field is applied in a z'-direction; a plurality of electrical
contacts coupled to each of the plurality of optical waveguides;
and an electrical field source configured to provide the external
electric field in the z'-direction at each of the plurality of
optical waveguides, via the plurality of electrical contacts.
10. The communication system of claim 9, wherein the index of
refraction of each of the plurality of optical waveguides is
variable by application of the external electric field.
11. A communications system, comprising: a light source configured
to emit light having a predetermined wavelength; a polarization
apparatus configured to prepare light from the light source into
particular states of polarization, the polarization apparatus
comprising: a first, fixed linear polarizer, a 90 degree optical
retarder, and a rotatable, second linear polarizer; an
electro-optical light phase modulator configured to produce light
having a plurality of wavelengths and configured to retard the
phase of propagation of light with a first state of linear
polarization while not retarding the phase of light with a state of
linear polarization orthogonal to the first state of linear
polarization when an external electric field is applied; and a
sinusoidal electrical oscillator configured to periodically apply
an electric field to the light phase modulator.
12. The communication system of claim 11, wherein the light phase
modulator is configured to produce a first light characterized by a
first wavelength and first state of linear polarization and a
second light characterized by a second wavelength and a second
state of polarization orthogonal to the first state of
polarization.
13. The communication system of claim 11, wherein the light phase
modulator comprises an electro-optic crystal belonging to space
group symmetry F43m and configured as a Mach-Zehnder
interferometer.
14. The communication system of claim 13, wherein the electro-optic
crystal belongs to the class of III-V compound semiconductors.
15. The communication system of claim 11, further comprising a
light flux modulator configured to change the flux of light with a
first state of linear polarization while not changing the flux of
light with a state of linear polarization orthogonal to the first
state of linear polarization when an external electric field is
applied.
16. The communication system of claim 11, wherein the light flux
modulator comprises an electro-optic crystal belonging to space
group symmetry F43m and configured as a Mach-Zehnder
interferometer.
17. The communication system of claim 15, wherein the light flux
modulator is configured to encode data on light characterized by a
first state of polarization while not encode data on light
characterized by a second state of polarization orthogonal to the
first state of polarization.
18. The communication system of claim 15, wherein the light flux
modulator comprises: a plurality of optical waveguides extending
parallel to a crystal surface having a perpendicular axis parallel
to the z-axis and configured and oriented such that light
propagates in a waveguide parallel to a y'-direction or a
x'-direction while an external electric is applied in a
z'-direction; a plurality of electrical contacts coupled to each of
the plurality of optical waveguides; and an electrical field source
configured to provide the external electric field in the
z'-direction at each of the plurality of optical waveguides, via
the plurality of electrical contacts.
19. The communication system of claim 18, wherein the plurality of
optical waveguides comprise buried waveguides or ridge
waveguides.
20. The communication system of claim 18, wherein the electric
field source is configured to be substantially periodic in
time.
21. A communications system, comprising: a light source configured
to emit light having a predetermined wavelength; a polarization
apparatus configured to prepare light from the light source into
particular states of polarization, the polarization apparatus
comprising: a first, fixed linear polarizer, a 90 degree optical
retarder, and a rotatable, second linear polarizer; a phase
modulator configured to produce light having a plurality of
wavelengths and configured to retard the phase of propagation of
light with a first state of linear polarization while not retarding
the phase of light with a state of linear polarization orthogonal
to the first state of linear polarization when an external electric
field is applied; an electrical oscillator configured to
periodically apply an electric field to the phase modulator; and a
light flux modulator configured to change the flux of light with a
first state of linear polarization while not changing the flux of
light with a state of linear polarization orthogonal to the first
state of linear polarization when an external electric field is
applied.
22. The communication system of claim 21, wherein the light flux
modulator is configured to encode data on light characterized by a
first state of polarization while not encode data on light
characterized by a second state of polarization orthogonal to the
first state of polarization.
23. The communication system of claim 21, wherein the light flux
modulator comprises: a plurality of optical waveguides extending
parallel to a crystal surface perpendicular to the crystal
z-direction waveguides configured and oriented such that light
propagates in the waveguides in a y'-direction or a x'-direction
while an external electric field is applied in a z'-direction; a
plurality of electrical contacts coupled to each of the plurality
of optical waveguides; and an electrical field source configured to
provide the external electric field in the z'-direction at each of
the plurality of optical waveguides, via the plurality of
electrical contacts.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims priority to and benefit under 35
U.S.C .sctn.119(e) of U.S. Provisional Patent Application Ser. No.
62/116,069 filed Feb. 13, 2015, which is hereby incorporated by
reference herein in its entirety as if fully set forth below.
[0002] Some references, which may include patents, patent
applications, and various publications, are cited in a reference
list and discussed in the disclosure provided herein. The citation
and/or discussion of such references is provided merely to clarify
the description of the present disclosure and is not an admission
that any such reference is "prior art" to any aspects of the
present disclosure described herein. All references cited and
discussed in this specification are incorporated herein by
reference in their entireties and to the same extent as if each
reference was individually incorporated by reference.
BACKGROUND
[0003] As optical fiber and optical-wireless communication network
advance to accommodate fifth generation wireless networks,
dual-polarization coherent optical communication schemes are being
widely envisioned for use in trunk networks and deep wavelength
division multiplexing (DWDM) networks to increase spectral and
power efficiency ([Roberts 2009]). Greater control over phase
coherence is advantageous in wireless networks that carry radio
frequency services from a central office to a radio transmitter
antenna by utilizing a local optical fiber network within a macro
cell. By harnessing dual polarization as an orthogonal modulation
scheme, all degrees of freedom of the light wave, i.e, amplitude,
phase, and polarization, can be utilized simultaneously for the
conveyance of data ([Li 2009], [Nakazawa 2010]).
[0004] Phase coded information cannot be detected directly by a
photodetector. A CW light source having a narrow optical spectrum
is required to act as an optical local oscillator such that when
mixed with the data bearing optical signal on balanced
photo-detectors, the amplitude and phase of the coded signal is
retrieved by virtue of the linear response of the photo-detector to
the incident fields and by using digital processing techniques. The
phase coded information is down-converted from the optical domain
to the electrical domain by virtue of the interference beating of
electric fields from both the signal light and an optical local
oscillator light on the photo-detector. This process is commonly
referred to as "heterodyne coherent detection" ([Ip 2008]). In
order to obtain reliable data down-conversion from a complex
carrier wave exhibiting high order modulation, random phase
fluctuations, and random polarization fluctuations in both the
local oscillator and signal lights, the phase of the local
oscillator light must be controlled to a high degree; better yet if
the phase of the signal light were correlated to the phase of the
optical local oscillator and the optical spectrum of each were to
be very narrow. In general this is difficult to achieve with
un-correlated or free-running optical local oscillators and signal
light sources, particularly after several kilometers of optical
fiber. Consequently, various ways to digitally retrieve phase
information are conventionally employed. The complexity of digital
data processing can be simplified if the phase of the local
oscillator light were to be derived from the same light source as
the signal light.
[0005] Disadvantages of some prior art approaches, including
complexity and cost of optical coherent receiver equipment, will
now be described. The problem of phase noise is, in part, remedied
by the use of optical local oscillator sources having a narrow
optical spectrum, used to down-convert baseband data from the
optical domain to the electrical domain. This is followed by
complex digital signal processing integrated circuits and
algorithms to equalize received signals, maintain phase coherence,
and partially compensate for random fluctuations. Digital data
processing technology in coherent optical signal detection is
described by Savory and Kuschnerov ([Savory 2010], [Kuschnerov
2009]). As shown in FIG. 1 (prior art), in the case of dual
polarization coherent detection, there are four receiver channels
for each polarization channel or one phase and one quadrature
channel for each of two orthogonal polarizations. Therefore, two
high speed, analog-to-digital converters are needed for each
polarization channel, one after each analog coherent receiver, to
convert the analog received signal to the digital domain. The
analog-to-digital converters must then be interfaced to a digital
signal processor unit which performs correction algorithms such as
chromatic dispersion compensation, polarization control and
equalization, carrier phase recovery, and forward error correction
decoding.
[0006] To meet the demand of growing data traffic, coherent
detection was introduced in Ultra-Dense Wavelength-Division
Multiplexing Passive Optical Networks (UDWDM-PON), as it promotes
high transmission capacity with enhanced spectral efficiency.
([Dong 2011], [Zhu 2012]). However, considering the cost, latency,
and power consumption attributed to spectrally narrow optical local
oscillators and digital signal processing (DSP) decoders, it may be
difficult to deploy DSP-based detection in a passive optical
network (PON), since in a PON architecture, the optical network
units (ONUs), that convert the received optical signal to
electrical signal, are located at the subscriber's premises, a
location that is not under the control of the service provider. ONU
environmental conditions vary and adjustments and maintenance
cannot be shared with the subscriber. Thus, ONUs have to be simple,
reliable and not require tuning or maintenance. Two potentially
cost effective ways to render ONUs suitable for the UDWDM-PON
network are: 1) replace the narrow-linewidth optical local
oscillator (LO) in the ONU with a cost-effective alternative; and
2) reduce the hardware implementation complexity of the DSP unit.
([Presi 2014] and [Prat 2012]).
[0007] In prior art coherent heterodyne detection, a weak
information bearing optical signal and a substantially stronger
continuous wave local optical oscillator light of somewhat
different but spectrally narrow optical wavelength may be mixed on
a photodetector to retrieve data using received power at sum and
difference frequencies, enhanced in magnitude by the stronger
amplitude of the optical local oscillator. To reduce ONU cost
further, a single polarization, self homodyne, optical
communication link that does not use an optical local oscillator
and convert data to the electrical domain by direct conversion on a
photodetector has been reported. ([Shahpari 2014]). However, an
external cavity laser is used at the transmitter along with
additional optical filtering at the receiver. DSP and complex
signal processing algorithms, for example, analog-to-digital
converters (ADCs), forward error correction, static equalizer,
phase recovery estimation, and dynamic equalizer, are still
necessary for phase and polarization estimation. Cost reduction has
consequences: slower ADCs can be used at the expense of
under-sampling of the received signal, use of serial-to-parallel
converters and increased filter complexity in the DSP unit.
Unfortunately, only low order modulation formats with a single
polarization mode have been demonstrated by using these
methods.
[0008] FIG. 1 shows a prior art system 100 generally comprising a
coherent optical detection scheme operating in dual polarization
mode with in-phase and quadrature coding. A polarized light stream
carries encoded data (see "DATA"). Orthogonal polarization
components are separated by polarizer 105. A spectrally narrow
external cavity laser 115 provides a local optical oscillator
reference. An optical light splitter 120 divides the reference
light into two paths to be combined with each of the two orthogonal
polarizations by 90.degree. optical hybrids 125. The composite
light streams containing data and reference light streams are
incident on pairs of balanced photo-diode detectors 130 where
electric fields of each light stream are mixed by the non-linear
response of the photo-diode detectors that generate a corresponding
electrical waveform response representative of the in-phase and
quadrature data carried by each state of polarization. The
electrical response of each photo-diode detector pair is amplified
by amplifiers 135 and digitally analyzed and processed by digital
signal processing apparatus 110, which may perform signal
correction and data recovery functions, examples of which can be:
analog-to-digital conversion, channel equalization, polarization
de-multiplexing, polarization mode dispersion compensation, clock
recovery, phase recovery and estimation and quadrature phase shift
key decoding. Much of the DSP equipment is used to compensate for
channel impediments of which polarization mode dispersion,
chromatic dispersion, and phase de-coherence are primary
manifestations. It is desirable to reduce the magnitude of channel
impediments so as to reduce the amount of DSP resources necessary
to retrieve base band data.
[0009] Disadvantages of prior art means for generation of
orthogonally polarized lights and information encoding thereof will
now be described. Modulation formats are a key part of
communication in that they enable spectrally efficient wireless and
wired communication. When communicating over optical fibers,
optical single sideband phase modulation has been shown to reduce
unwanted chromatic dispersive effects on the light carrier wave.
Optical fiber can provide long distance transportation of wireless
information. Radio frequency information can be converted to the
optical domain on optical sidebands of an optical carrier
wavelength and transported over optical fibers and subsequently
converted back to the electrical domain to propagate wirelessly.
The preservation of data, specifically phase information, upon
transition from optical fiber to free space electromagnetic wave
propagation, requires that a coherent phase relation be maintained
between the carrier frequency and the single sideband frequency.
Relative phase or wavelength variations, for example spectral
broadening in the optical domain, directly translate into radio
frequency noise, signal fading of free-space radio waves, and loss
of data integrity.
[0010] When a robust coherent phase relation exists between an
optical carrier frequency having electric field oriented in a first
direction and derived single or double sideband frequency or
frequencies having electric field oriented in an orthogonal
direction, the process of coherent heterodyne detection can be
simplified without referring to an external optical local
oscillator. To maintain strong phase coherence, the carrier
frequency and sideband frequencies can originate from the same
narrow laser source and both traverse the same optical path through
various optical components, and electro-optic modulators in
particular.
[0011] Further, the carrier frequency and optical sideband(s), in
addition to being spectrally separated, can be orthogonally
polarized relative to one another, as they propagate along the same
optical channel. In this way, an isotropic channel will
substantially present the same impediment mechanisms to both
carrier and signal sidebands. In contrast, a crystal modulator
generally presents anisotropic optical properties that depend on
the polarization direction of light and direction of propagation
with respect to a crystal axis of symmetry. For example, light that
is polarized along a first crystal direction will be maximally
modulated by a LiNbO.sub.3 electro-optic modulator while light that
is polarized in an orthogonal direction will be substantially less
modulated due to the intrinsic birefringence of the LiNbO.sub.3
crystal. Other electro-optic crystals such as GaAs or InP are not
intrinsically birefringent and can lead to the case in which light
is modulated in a first state of polarization while un-modulated in
the orthogonal state of polarization. This particular property of
naturally non-birefringent or isotropic crystal electro-optic
modulators can be important in preventing modulation leakage
between two orthogonally polarized channels.
[0012] Various prior art methods have been demonstrated that are
capable of producing co-linear light streams that differ in
wavelength and are orthogonally polarized relative to one another.
However, these methods produce lights that are not strongly
correlated in phase and therefore suffer from random noise, signal
fading of free-space radio waves, and loss of data integrity. One
method to produce lights having different wavelengths and
orthogonal states of polarization is described by Sagues, et al.
([Sagues 2010]), which makes use of stimulated Brillouin scattering
in an optically pumped optical fiber. Two parallel polarized light
waves differing in wavelength are phase coherent and have spectral
separation greater than the Brillouin linewidth. The Brillouin
linewidth in silica glass is typically 130-210 MHz at a pump
wavelength of 4880 .ANG.. The optical fiber has low chromatic
dispersion. A counter propagating pump light is polarized
perpendicular to the polarization direction of two parallel
polarized lights. One of the wavelength pair is chosen to fall
within the Brillouin linewidth and its linear polarization
gradually rotates toward the polarization direction of the pump
light, while the second wavelength of the pair is chosen to lie
outside the Brillouin bandwidth and its polarization remains
unchanged. The technique uses an optical circulator connecting the
pump light, the two parallel polarized incident wavelengths and the
two orthogonally polarized exiting wavelengths, neither of which is
modulated to convey information. If any one of the exiting lights
were to be encoded with data, it would have to be diverted to a
modulator and consequently follow a different path. In that case,
its phase correlation with respect to its twin, un-modulated light,
can no longer be assured.
[0013] Another prior art method that results in light streams
having different wavelengths and mutually orthogonal polarizations
is described by Campillo. ([Campillo 2007]). The method uses a
polarization modulation crystal waveguide by means of which an
initial light stream having a first wavelength and a first
polarization is converted to two exiting light streams: one
comprising a portion of the incident light with initial
polarization, and an orthogonally polarized sideband having a
second wavelength. The sideband carries no information. The
introduction of an output polarizer can provide intensity on-off
modulation that can be configured to convey information.
[0014] Another prior art method of producing lights having
different wavelengths, at least two of which are orthogonally
polarized relative to one another, is described by Zheng, et al.
([Zheng November 2014]). This technique uses a Sagnac loop
interferometer, a double drive Mach-Zehnder modulator and a
polarization maintaining Bragg grating optical fiber to convert an
incident light stream having a first wavelength and a first
polarization to an exiting light stream having the same spectral
content and polarization as the incident light but having reduced
intensity. An orthogonally polarized sideband is produced in the
process, comprising a second wavelength. If any one of the exit
light waves were to be encoded with data, it would have to be
separated and consequently follow a different path. In that case
its phase correlation to the un-modulated wavelength can no longer
be assured.
[0015] Disadvantages of prior art approaches with LiNbO.sub.3
birefringent modulators will now be described. To date, the most
common electro-optic modulator in use in telecommunication is the
lithium niobate (LiNbO.sub.3), abbreviated as LN, modulator. The
LiNbO.sub.3 crystal displays trigonal crystal symmetry (space group
symmetry R3c) and is intrinsically birefringent with index of
refraction having the uniaxial form: n.sub.o=n.sub.x=n.sub.y=2.297
and n.sub.o=n.sub.z=2.208. Its linear electro-optic tensor
coefficients are: r.sub.13=8.6.times.10.sup.-12 m/V,
r.sub.22=3.4.times.10.sup.-12 m/V, r.sub.33=30.8.times.10.sup.-12
M/V and r.sub.51=28.0.times.10.sup.-12 m/V. Since r.sub.33, along
the extraordinary axis of LN, is the largest electro-optic
coefficient, an electric field, F.sub.j (j=x, y, z), applied
parallel to the extraordinary axis (z-direction of the index
ellipsoid) will result in the most efficient modulation. Therefore,
under the external electric field: F.sub.z.noteq.0 and
F.sub.x=F.sub.y=0, the index ellipsoid for LiNbO.sub.3 can be
represented by:
x 2 ( 1 n o 2 + r 13 F z ) + y 2 ( 1 n o 2 + r 13 F z ) + z 2 ( 1 n
o 2 + r 33 F z ) = 1 Eq . 1 ##EQU00001##
The z-direction is that of the extra-ordinary crystal axis in
uniaxial LN.
[0016] In the case of an x-cut LN crystal, an external electric
field F.sub.z applied along the z-direction lies in the plane of
the crystal surface. Prior art electrode configurations are
illustrated FIG. 2, which depicts a cross section drawing of an
x-cut LiNbO.sub.3 electro-optic modulator 200(a) and an electrode
configuration in the case of a z-cut LiNbO.sub.3 crystal
electro-optic modulator 200 (b). Features 220 and 215 represent
ground (G) and signal (S) electrical contacts separated from the
crystal by buffer layer 225. Electric field lines F.sub.z are
represented by features 230 and are oriented predominantly along
the z-direction at the waveguide core 235, or the direction of the
extraordinary axis of the crystal. TE and TM waveguide modes are
both supported by the dielectric rectangular waveguides in LN.
([Wooten 2000]).
[0017] In the case of x-cut LN crystal 200(a), the optical
waveguide is oriented along the y-axis (because the x-axis is
vertical to the LN wafer surface and the z-axis is the direction of
the applied electric field). Therefore, for light polarized with
electric field along the x-axis or z-axis, the optical refractive
indices are given by:
n.sub.x.apprxeq.n.sub.y=n.sub.o-1/2n.sub.o.sup.3r.sub.13F.sub.z Eq.
2
n.sub.z.apprxeq.n.sub.e-1/2n.sub.e.sup.3r.sub.33F.sub.z Eq. 3
[0018] Therefore, under an external modulation electrical field
applied in the z-direction, the LiNbO.sub.3 crystal remains
uniaxial and the optical axis remains unchanged, but the index
ellipsoid is deformed by the modulation field, F.sub.z, in
accordance to Eqs. 1 and 2. Light propagating along the z-direction
will experience the same phase change, independent of polarization.
However, light propagating along the x- or y-direction will
experience a phase change on its state of polarization. In the case
of an x-cut LiNbO.sub.3 crystal, both electrodes are placed
symmetrically on both sides of the waveguide such that the bias
field is along the z-direction. In this case, if light is
propagating along a waveguide aligned with the y-direction and is
polarized along the z- (or x-) direction, then the electric field
components will be modulated in accordance with Eq. 4 (or Eq. 5),
where E.sub.TE, E.sub.TM, and E.sub.o refer to the electric field
amplitude of the light.
E.sub.TE={circumflex over
(z)}E.sub.oe.sup.-ik.sup.o.sup.(n.sup.e.sup.-1/2n.sup.e.sup.3.sup.r.sup.3-
3.sup.F.sup.z.sup.)y Eq. 4
E.sub.TM={circumflex over
(x)}E.sub.oe.sup.-ik.sup.o.sup.(n.sup.e.sup.-1/2n.sup.e.sup.3.sup.r.sup.1-
3.sup.F.sup.z.sup.)y Eq. 5
Equations 4 and 5 show that for the x-cut LiNbO.sub.3 crystal
modulator, the TE optical mode (polarized along the z-axis) is more
efficiently modulated than the TM optical mode (polarized along the
x-axis) because r.sub.33 is greater than r.sub.13
(r.sub.33/r.sub.13=3.58), resulting in a TE/TM power extinction
ratio of about 20 dB, clearly contaminating the orthogonally
polarized channel.
[0019] In the case of a z-cut LiNbO.sub.3 crystal modulator 200(b),
an electric field applied along the z-direction means that the
electric field is vertical. In this case, the waveguide can be
defined along either the x- or y-direction. For example, for an
optical waveguide fabricated along the y-axis, the intersection
ellipse is again represented by Eq. 1, and the optical indices of
refraction are the same as those in Eq. 2 and Eq. 3. However, for a
z-cut crystal, the TM optical mode is polarized along the z-axis,
and the TE optical mode is polarized along the x-axis.
Consequently, for the z-cut LiNbO.sub.3 modulator, the TM mode is
more efficiently modulated than the TE mode, by the same power
ratio of about 20 dB, clearly contaminating the orthogonally
polarized channel.
[0020] Conventional approaches exist for light streams containing
plural wavelengths, at least two of which display electric fields
that oscillate along orthogonal directions, are co-linear and
correlated in phase. However, there does not exist any teaching on
how to maintain phase coherence and orthogonal polarization while
at the same time encode information on at least one wavelength
channel in the light stream, or how to encode different information
on two orthogonally polarization channels having the same
wavelength as is necessary for in phase and quadrature coding.
[0021] It is with respect to these and other considerations that
the various embodiments described below are presented.
SUMMARY
[0022] Some aspects of the present disclosure relate to apparatus
generating phase-correlated orthogonally polarized lights, method
of construction, and fiber wireless communication systems based
thereon.
[0023] Some aspects of the present disclosure relate to a
phase-correlated, orthogonally-polarized, light-stream generator
(POLG) apparatus and method of construction and use. In some
embodiments, by means of the POLG, the state of a stream of light
may be constructed from a single light source. The stream of light
may contain a plurality of wavelength channels, at least two of
which can be linearly polarized orthogonal to one another. At least
one optical channel can be modulated without disturbing
orthogonally polarized channels. All optical channels can be
substantially co-linear and carried in optical fibers. All
wavelength channels can maintain coherent phase relation with
respect to one another and two orthogonally polarized,
phase-coherent channels can be individually coded and configured to
communicate in phase and quadrature code by means of radio
frequency without using radio frequency mixers.
[0024] Some aspects of the present disclosure relate to the
preparation of light streams that are orthogonally polarized,
contain multiple wavelengths, and whose phases of propagation are
correlated to one another. Some aspects further relate to encoding
data on one light stream without perceptibly disturbing
orthogonally polarized light streams.
[0025] Some aspects of the present disclosure relate to the
generation of coded radio frequency waves from spectrally different
and phase correlated light streams, at least one of which carries
coded information. Some aspects of the present disclosure relate to
the generation of phase correlated, radio frequency local
oscillator signals, derived from phase correlated light streams. In
some embodiments, radio frequency local oscillators can be used to
demodulate received radio frequency transmissions.
[0026] Some aspects of the present disclosure relate to analog
in-phase and quadrature modulation of radio frequency waves. In
some embodiments, phase and quadrature data can be derived from
phase correlated light streams and are encoded on orthogonally
polarized lights. Some aspects of the present disclosure relate to
a co-propagating central carrier using phase correlated local
oscillator in digital signal processing front end in coherent
optical signal detection.
[0027] In accordance with some aspects of the present disclosure,
in one or more embodiments, a communication system can include a
first section for preparing the state of polarization of a POLG,
and a second section. The first section may incorporate a
semiconductor light source, a linear polarizer, a 90.degree.
optical retardation plate, and a second linear polarizer that can
be rotated. Additionally, the first section may include an
electrically-driven phase modulator that periodically retards the
phase of propagation of a light stream having a first polarization
while not perceptibly affecting light having orthogonal
polarization. In the second section, data may be encoded on a light
stream having a first linear polarization while a light stream
having orthogonal polarization may be not perceptibly affected.
[0028] In accordance with some aspects of the present disclosure,
in one or more embodiments, semiconductor crystals exhibiting the
zinc blende structure with space group symmetry F43m are used in
communication systems. Gallium arsenide and indium phosphide are
two representative semiconductors exhibiting this space group.
Optical waveguides composed of these semiconductors can be oriented
along [110] or [110] crystallographic direction in a (001) plane
and may be activated by electric fields substantially oriented in
the [001] direction while light propagates in the optical
waveguides configured as an optical interferometer. The
interactions between the applied external electric field in the
[001] direction, the symmetry of the F43m electro-optic tensor and
the orientation of the waveguides parallel to [110] or [110]
direction in the (001) plane may be such that only light that is
polarized with electric field vector in the (001) plane is
modulated by the applied field, while light that is polarized with
electric filed vector parallel to the [001] direction is not
perceptibly modulated.
[0029] In accordance with some aspects of the present disclosure,
one or more embodiments may comprise a radio frequency transmitter
and a radio frequency receiver, both having functionalities enabled
by the state of polarization, phase coherence, spectral content of
the light streams prepared by a POLG, and the polarization-specific
selectivity of semiconductor modulator crystals exhibiting F43m
symmetry. A first light stream comprising a data-bearing sideband
and a second, un-modulated central frequency can be prepared in a
first state of polarization in conjunction with the POLG. The first
light stream can generate data coded radio frequency waves by
heterodyne mixing of the optical fields on a semiconductor
photo-diode. The coded radio frequency waves can be launched into
free space by a radio frequency antenna radiator. The second light
stream, simultaneously prepared by the POLG, can be polarized
orthogonal to the first light stream, and has a central carrier
frequency and at least one non-modulated sideband frequency. The
second light stream generates a local oscillator, radio frequency
signal by heterodyne mixing of the optical fields on a second
semiconductor photo-diode. The local oscillator signal can be used
in conjunction with a phase tracking feed-back loop and a radio
frequency mixer to retrieve data received by radio frequency
transmission.
[0030] In in-phase and quadrature communication, both phase and
quadrature amplitude may be parceled into finer increments in a
symbol cycle, sequence the phase and amplitude increments upon
transmission and retrieve the same sequences upon reception. This
practice is commonly referred to as high order format coding and
makes use of analog in-phase and quadrature modulation, either of
light streams in wired communication or of radio frequency waves in
wireless communication.
[0031] In accordance with another aspect, in some embodiments of
the present disclosure, phase and quadrature relations can be
derived from the POLG in conjunction with subsequent modulation
with electro-optic modulators exhibiting F43m space group symmetry
and re-arrangements of the state of polarization of light stream.
Such embodiments can make use of a polarization-maintaining optical
coupler, a 90.degree. symbol optical delay, first and second
electro-optic modulators comprising crystals exhibiting F43m space
group symmetry, and a polarization combiner that re-constitutes the
in-phase and quadrature of received coded light streams. A
reconstituted received light stream may be used, in conjunction
with heterodyne mixing on a suitable photo-diode and a radio
frequency antenna radiator, to transmit high format modulated data
on radio frequency carrier waves.
[0032] In accordance with some aspects of the present disclosure,
one or more embodiments can comprise co-propagating central carrier
wavelength and sideband data in each of two orthogonal light
streams, where one carries in-phase data while the other carries
quadrature data. The two orthogonally polarized light streams can
be sent, each to heterodyne mix central frequency with data
sideband and amplified before entering the digital signal
processing stage. The detection process is simplified by the phase
coherent, co-propagating reference wave.
[0033] In accordance with some aspects of the present disclosure,
one or more embodiments can utilize an electro-optic modulator
comprising optical waveguides with zinc blende crystal symmetry by
means of which traversing light of one polarization can be
substantially modulated while light of the orthogonal polarization
is substantially not effected and is used as the phase correlated,
local optical oscillator in optical coherent detection simplifying
digital signal processing at the receiver, thereby simplifying
digital signal processing equipment and minimizing latency
time.
[0034] In accordance with some aspects of the present disclosure,
in one or more embodiments, a POLG apparatus can comprise RF
Encryption. When used in conjunction with polarization selective
modulators, the POLG apparatus can use optical methods to transmit
analog radio frequency data in a format generally referred to as
"frequency hopping spread spectrum" relating to the transmitting
and receiving of secure data in wireless radio frequency
communications.
[0035] Other aspects and features according to the present
disclosure will become apparent to those of ordinary skill in the
art, upon reviewing the following detailed description in
conjunction with the accompanying figures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0036] Reference will now be made to the accompanying drawings,
which are not necessarily drawn to scale.
[0037] FIG. 1 illustrates a prior art system with a coherent
optical detection scheme operating in dual polarization mode with
in-phase and quadrature coding.
[0038] FIG. 2 is a cross sectional view of prior art x-cut and
z-cut lithium niobate crystal modulators with sketched external
drive field directions, electrical contact placement, and optical
waveguide orientation.
[0039] FIG. 3 illustrates a phase-correlated orthogonally-polarized
light-stream generator (POLG) apparatus and data coding apparatus,
with a corresponding sketch of the state of polarization and
frequency spectrum at designated points along the optical path, in
accordance with one embodiment of the present disclosure.
[0040] FIG. 4 is a cross sectional view of an electro-optic
modulator comprising crystal exhibiting F43m space group symmetry,
showing external applied electric field direction, electrical
contact placement, and optical waveguide orientation designated by
Miller indices for the space group F43m.
[0041] FIG. 5 illustrates a prior art index ellipsoid for an
electro-optically active crystal having zincblende symmetry,
depicting external electric field effect on the index of
refraction.
[0042] FIG. 6 illustrates zinc blende crystal waveguides on the
(001) surface and oriented parallel to the [110] direction and
configured to function as a light flux modulator for TM polarized
light while affecting no modulation on TE polarized light, in
accordance with one embodiment of the present disclosure.
[0043] FIG. 7 illustrates an apparatus transmitting optically coded
radio frequency data and providing radio frequency local oscillator
for analog decoding received radio frequency waves, in accordance
with one embodiment of the present disclosure.
[0044] FIG. 8 illustrates an apparatus for preparing in-phase and
quadrature coded radio frequency carrier waves in the optical
domain and apparatus for launching in-phase and quadrature coded
radio frequency carrier wave into free space, in accordance one
embodiment of the present disclosure.
[0045] FIG. 9 shows spectral content, state of polarization, and
radio frequency phase at various stages during the light stream
preparation process in the optical domain, in accordance with some
embodiments of the present disclosure.
[0046] FIG. 10 illustrates a system with an optical coherent
detection sub-assembly without a separate optical local oscillator,
in accordance with one embodiment of the present disclosure.
[0047] FIG. 11 illustrates a system using a POLG and polarization
modulators for transmitting and receiving of secure data in
wireless radio frequency communications.
DETAILED DESCRIPTION
[0048] Although example embodiments of the present disclosure are
explained in detail herein, it is to be understood that other
embodiments are contemplated. Accordingly, it is not intended that
the present disclosure be limited in its scope to the details of
construction and arrangement of components set forth in the
following description or illustrated in the drawings. The present
disclosure is capable of other embodiments and of being practiced
or carried out in various ways.
[0049] It must also be noted that, as used in the specification and
the appended claims, the singular forms "a," "an" and "the" include
plural referents unless the context clearly dictates otherwise.
Ranges may be expressed herein as from "about" or "approximately"
one particular value and/or to "about" or "approximately" another
particular value. When such a range is expressed, other exemplary
embodiments include from the one particular value and/or to the
other particular value.
[0050] By "comprising" or "containing" or "including" is meant that
at least the named compound, element, particle, or method step is
present in the composition or article or method, but does not
exclude the presence of other compounds, materials, particles,
method steps, even if the other such compounds, material,
particles, method steps have the same function as what is
named.
[0051] In describing example embodiments, terminology will be
resorted to for the sake of clarity. It is intended that each term
contemplates its broadest meaning as understood by those skilled in
the art and includes all technical equivalents that operate in a
similar manner to accomplish a similar purpose. It is also to be
understood that the mention of one or more steps of a method does
not preclude the presence of additional method steps or intervening
method steps between those steps expressly identified. Steps of a
method may be performed in a different order than those described
herein without departing from the scope of the present disclosure.
Similarly, it is also to be understood that the mention of one or
more components in a device or system does not preclude the
presence of additional components or intervening components between
those components expressly identified.
[0052] In one embodiment of the present disclosure, a
phase-correlated orthogonally-polarized light-stream generator
(POLG) apparatus is configured to prepare light streams displaying
plural phase-correlated wavelengths that are either parallel or
orthogonally polarized with respect to one another.
[0053] In one embodiment of the present disclosure, an
electro-optic modulator having optical waveguides with zinc blende
crystal symmetry is configured to substantially modulate a
traversing light of one polarization while not affecting a light of
an orthogonal polarization. In one embodiment, the electro-optic
modulator can be used as a phase-correlated, local optical
oscillator in optical coherent detection simplifying digital signal
processing at the receiver, thereby simplifying digital signal
processing equipment and minimizing latency time.
[0054] In one embodiment, a light stream having orthogonally
polarized lights traverses a zinc blende electro-optic modulator
wherein a first polarized light can be encoded with data while a
second orthogonally polarized light can be un-modulated. In one
embodiment, a first data bearing light stream is converted to radio
frequency data transmission while a second, orthogonally polarized,
light stream generates a radio frequency local oscillator signal
used in conjunction is radio frequency mixer and phase tracking
feed-back loop for decoding incoming radio frequency signal having
the same carrier frequency, thus simplifying the radio frequency
receiving apparatus.
[0055] One embodiment can use optical phase-sensitive, high format
modulation, e.g., an in-phase and quadrature phase shift key
modulation, and the conversion of the formatted light stream to
in-phase and quadrature radio frequency electrical signals that can
be radiated as an in-phase and quadrature carrier wave without the
need for radio frequency mixers or frequency synthesized electrical
local oscillator.
[0056] One embodiment can use detection of high format modulated
optical signals having orthogonal, linearly polarized and
phase-coherent light streams, where a first stream carries in-phase
data and a second stream carries quadrature data, and is processed
by digital processing hardware and algorithms. The processing by
digital processing hardware can be simplified and comprise
increased coherency by virtue of the phase-coherent state and
orthogonal polarization of the constituent light streams.
[0057] In one embodiment, a POLG apparatus can comprise RF
Encryption. When used in conjunction with polarization selective
modulators, the POLG apparatus can use optical methods to transmit
analog radio frequency data in a format generally referred to as
"frequency hopping spread spectrum" relating to the transmitting
and receiving of secure data in wireless radio frequency
communications.
[0058] In one embodiment, a coherent communication system can
comprise two apparatus: a POLG configured to prepare a stream of
light in a state having plural wavelengths, at least two of which
are linearly polarized orthogonal to one another; and an
electro-optic light flux modulator having one or more optical
waveguides belonging to a crystalline material displaying zinc
blende or F43m space group symmetry and configured to modulate only
one of two orthogonal states of linear polarization of light while
imperceptibly affecting the orthogonal state of polarization.
[0059] Some embodiments of the present disclosure can comprise a
POLG sub-assembly and a highly polarization selective electro-optic
light modulator. One embodiment can comprise both a transmitter and
a receiver of radio frequency signal. Another embodiment can
comprise a phase and quadrature modulation means for coding both
optical and radio frequency transmissions. Another embodiment can
comprise simplified in phase and quadrature optical coherent
detection without use of an external optical oscillator.
[0060] In one embodiment, transmission of optical radio frequency
information and receiving and processing of information carried by
electromagnetic, radio frequency waves can be simplified. Both
radio frequency transmission and reception can be enabled by the
POLG and polarization specific modulators.
[0061] FIG. 3 illustrates a system including a POLG 305, in
accordance with one embodiment of the present disclosure. The
system of FIG. 3 is configured for producing a light stream
simultaneously comprising one carrier wavelength .lamda..sub.0
having a first linear polarization direction and two principal
optical sidebands .lamda..sup.+1 and .lamda..sup.-1 having a second
linear polarization orthogonal to the first. Wavelength sidebands
and polarization result when light of wavelength .lamda..sub.0
traverses phase modulator 320, operated at cyclic frequency f.
Phase modulator 320 comprises one or more waveguides having zinc
blend crystal symmetry and can be configured to modulate only one
of two orthogonal states of linear polarization of light while
imperceptibly affecting the orthogonal state of polarization. As
shown at 300(b) each sideband .lamda..sup.+1 and .lamda..sup.-1 is
separated from the carrier wave .lamda..sub.0 by frequency interval
.delta.f=c/(.lamda..sub.0+.lamda..sup.+)=c/(.lamda..sub.0-.lamda..sup.-).
Light flux modulator 340 and phase modulator 320 are illustrated
further in FIGS. 4-6.
[0062] The POLG 305 can prepare light from a single source into
phase-coherent light streams having defined states of polarization
and spectral composition. Thus, in some embodiments, the POLG 305
may comprise "light pre-processing." Light of a chosen wavelength
exiting POLG light pre-processing may be encoded with data and
transmitted and eventually decoded at a receiver by coherent
detection using a reference light emanating from the POLG, for
example, .lamda..sub.0. Exemplary embodiments are illustrated in
FIGS. 7-9. Light pre-processing is advantageous because it can
assure a high degree of coherence, low degree of phase noise, and
auto-compensation for the channel impediment, commonly referred to
as polarization mode dispersion and polarization ellipticity. When
used in conjunction with a data coding device, for example an
electro-optic light flux modulator 340 (see 300(a) of FIG. 3),
leaking of information encoded in a channel into an orthogonal
channel can be prevented. For example, in some embodiments, the
electro-optic light flux modulator 340 can allow passage of two
orthogonal, linear polarized lights having different wavelengths,
and encode data on light of only one linear polarization, while
leaving undisturbed the co-propagating, perpendicularly polarized
light.
[0063] Referring again to 300(a) of FIG. 3, the POLG apparatus 305
may comprise a distributed feedback laser diode 310, a polarization
apparatus 315 to prepare the initial state of polarization of
light, and a sinusoidal electrical oscillator 325 that applies a
periodic electrical field to the electro-optical phase modulator
320. The polarization apparatus 315 may comprise fixed linear
polarizer (P), a (1/4.lamda.) retardation plate, and a rotating
linear polarizer (PR).
[0064] Referring now to 300(b) of FIG. 3, positions (1), (2), (3)
and (4) refer to corresponding numbered positions in drawing
300(a). As illustrated at 300(b), the state of linear polarization
and spectral components of the light stream are sketched at
positions along the path at 300(a). Light with wavelength
.lamda..sub.o emitted from laser diode 310 can traverse
polarization apparatus 315 and emerge in a desired state of linear
polarization with electric field vector making angle .alpha. with
respect to the principal axis of symmetry of the phase modulator
crystal 320, in this case, the y-axis. For example, in an
embodiment, the phase modulator crystal 320 can be LiNbO.sub.3 and
thereby become naturally birefringent. Thus, its modulation
efficiency can vary with polarization of the incident light stream,
and it is impossible to modulate light of one polarization and not
leak the modulation into the orthogonally polarized light.
Similarly, in an embodiment, the phase modulator crystal 320 can be
an intrinsically isotropic electro-optic crystal of zinc blende
symmetry which can be intentionally configured and operated in a
birefringent mode.
[0065] If the phase modulator crystal 320 is not operating in a
birefingent mode, then the modulation of light polarized in the
x-direction, for example, does not occur and only light that is
polarized in the y-direction is modulated, resulting in first order
optical sidebands at wavelengths .lamda..sup.-1 and .lamda..sup.+1.
Electro-optic crystals with space group symmetry F43m, for example
InP and GaAs, can be suitable crystalline materials that can be
configured as modulators exhibiting high polarization selectivity
based on the symmetry of their electro-optic tensor.
[0066] With input light .lamda..sub.0, the output of the POLG
apparatus 305 at 300(a) can comprise light at wavelength
.lamda..sub.0, polarized linearly in the x-direction, and optical
sideband lights at wavelengths .lamda..sup.+1 and .lamda..sup.-1,
polarized linearly in the y-direction. These lights can be used in
a coherent communication system. The orthogonally polarized optical
carrier with wavelength .lamda..sub.0 and the optical side bands
with wavelengths .lamda..sup.+1 and .lamda..sup.-1 can have
coherent phases since they originate from the same source and
traverse the same optical path. Furthermore, channel birefringence
and substantially stochastic polarization fluctuations in the
channel can affect all these lights in a substantially similar
manner during channel propagation, resulting in coherent,
substantially orthogonally polarized lights at the receiver.
[0067] As depicted at 300(a) and 300(b) of FIG. 3, an optical
bandpass filter 330 may be used to reject one optical sideband. The
remaining optical carrier .lamda..sub.0 and one optical sideband
.lamda..sup.-1 at position (3), for example, may traverse a
polarization-selective electro-optic light flux modulator 340
configured from a suitable crystal of space group symmetry F43m and
driven by data encoder 335. Polarization-selective light flux
modulator 340, can modulate light of one polarization, for example,
.lamda..sup.-1 with data sidebands 345 shown in 300(b) at position
(4), while not affecting orthogonally polarized light .lamda..sub.0
at position (4). Then the optical carrier light .lamda..sub.0
remains as the coherent, phase-correlated reference light which may
be used for coherent detection at the receiver, or for the optical
heterodyne generation of coded millimeter-waves for free-space
communication, or for upstream baseband communication. The degree
to which the light flux modulator 340 is polarization-selective can
depend on the natural properties of the electro-optic crystal, the
construction of the modulator and the propagation direction and
linear polarization direction of the lights with respect to the
crystal's axis of symmetry. Intrinsically birefringent crystals
such as Lithium Niobate (LiNbO.sub.3), Lithium Tantalate
(LiTaO.sub.3), and Potassium Titanyl Phosphate (KTiOPO.sub.4) are
not highly polarization selective and can result in partial
modulation of the orthogonally polarized channel. InP and GaAs are
not intrinsically birefringent and can result in no leakage of
modulation into the orthogonally polarized channel.
[0068] Aspects of polarization selective modulators in accordance
with various embodiments of the present disclosure will now be
described in further detail. Zinc Blende, III-V semiconductors,
space group symmetry F43m, are not intrinsically birefringent.
However, if subjected to an external electric field F.sub.z, the
index ellipsoid or indicatrix for a III-V semiconductor can take
the form
(x.sup.2+y.sup.2+z.sup.2)/n.sub.0.sup.2+2r.sub.41(yzF.sub.x+zxF.sub.y+xy-
F.sub.z)=1 Eq. 6
where n.sub.o is the ordinary refractive index, which is about 3.2
for InP, and r.sub.41 are the three non-zero, identical,
off-diagonal elements of the electro-optic tensor. Since crystal
growth in these materials usually proceeds perpendicular to the
(001) surface and along the [001] z-axis, in an exemplary
embodiment, an electric field can be applied along the z-axis as
represented in FIGS. 4 (a) and (b). In this case,
F.sub.x=F.sub.y=0; F.sub.z.apprxeq.0 and Eq. 6 can become
(x.sup.2+y.sup.2+z.sup.2)/n.sub.o.sup.2+2r.sub.41xyF.sub.z=1 Eq.
7
Due to the off-diagonal components of the electro-optic tensor, the
principal axes in the xy plane are rotated by 45.degree. in the
presence of an applied electric field F.sub.z, forming the rotated
coordinate system about the [001] (z-axis) with [110] (x'-axis) and
[110] (y'-axis) in the (001) plane. Therefore, the index ellipsoid
or the indicatrix in the new coordinate system can be represented
by Eq. 8, sketched in FIG. 4 at 400(a).
x'.sup.2[1/n.sub.o.sup.2+r.sub.41F.sub.z]+y'.sup.2[1/n.sub.o.sup.2-r.sub-
.41F.sub.z]+z.sup.2/n.sub.o.sup.2=1 Eq. 8
Eq. 8 shows that for a waveguide along [110] (x'-axis) or [110]
(y'-axis) direction, the index modulation can be given by
n'.sub.y'=[1/n.sub.o.sup.2-r.sub.41F.sub.z].sup.-1/2.apprxeq.n.sub.o+1/2-
n.sub.o.sup.3r.sub.41F.sub.z Eq. 9
n'.sub.x'=[1/n.sub.o.sup.2+r.sub.41F.sub.z].sup.-1/2.apprxeq.n.sub.o-1/2-
n.sub.o.sup.3r.sub.41F.sub.z Eq. 10
for the waveguide TE mode, which is polarized with electric field
in the crystal's z[001] direction, as depicted in FIG. 4 at 400(a).
One important consequence of Eq. 9 and Eq. 10 is that changes in
the refractive index only occur in the plane of the waveguide, the
xy-plane, when an external field F.sub.z is applied in the
orthogonal [001] z-direction. Therefore, for the waveguide
orientation shown in FIG. 4, the waveguide TM mode, which is
polarized with electric field parallel to the waveguide propagation
direction y' [110] is not modulated because all the electro-optic
tensor elements, aside from r.sub.41, are null. Therefore, all
III-V electro-optic modulators (e.g., InP/InGaAsP or GaAs/GaAlAs)
subject to the waveguide orientation depicted in FIG. 4 can have
high polarization selectivity under modulation in the z[001]
direction. Also note that the sign of the index change depends on
the waveguide orientation.
.DELTA.n=n.sub.y'-n.sub.x'=.+-.1/2n.sub.o.sup.3r.sub.41F.sub.z Eq.
11
[0069] An InP Mach-Zehnder modulator with waveguides also oriented
parallel to the [110] direction, but lying in the [110]-[001] or
(y-z) plane, not the (x-y) plane as depicted in FIG. 4, has been
described by Ogiso et al. ([Ogiso 2014]). The symmetry inherent in
Eq. 9 and Eq. 10, i.e., equal and opposite magnitude changes in the
index of refraction along directions [110] and [110] as described
by Eq. 9 and Eq. 10 and depicted in FIG. 4, is therefore broken,
enabling modulation of the TM mode in the case described by Ogiso
et al. ([Ogiso 2014]).
[0070] In FIG. 4, 400(a) depicts a cross section view of a zinc
blende crystal modulator constructed on a (001) surface of a
semi-insulating InP substrate 425, and comprising the substrate
425, an n-doped InP epitaxial layer 420, and a buffer layer 415
separating the semiconductor from ground electrical contacts (G)
405 and driving electrical contacts (S) 410, in accordance with an
embodiment of the present disclosure. Electric field lines produced
by the electrical contacts are depicted as features 430 and are
substantially oriented normal to the surface in the [001] direction
directly below the driving electrical contacts (S) where a light
guiding optical waveguide is formed by an InGaAsP region 435. The
optical waveguides 435 run parallel to a y' [110] crystal
direction. Polarization of the light stream is indicated by TM when
the magnetic field of the waveguide mode is oriented perpendicular
to the direction of the applied electric field, F.sub.z. TE
polarization can indicate that the waveguide mode has electric
field aligned perpendicular to the direction of the applied
electric field, F.sub.z.
[0071] An intuitive view of the effect of an applied external field
F.sub.z on the index of refraction of an electro-optic crystal
having space group symmetry F43m can be visualized by means of the
index ellipsoid of rotation construct about the direction of the
applied electric field, F.sub.z, usually referred as the z-axis
shown in FIG. 5, where the intrinsically isotropic index of
refraction is represented by a sphere outlined by the solid-line
505 and the index of refraction ellipsoid of rotation under the
action of applied field, F.sub.z, is represented by broken line
510. The direction of propagation of a electromagnetic plane is
represented by phase constant ({right arrow over (k)}) and the
isotropic refractive index when F.sub.z=0 is shown as n.
[0072] A polarization-selective modulator in accordance with
embodiments of the present disclosure can be constructed by
aligning optical waveguides and applying an external modulation
field in accordance with a crystal's electro-optic tensor symmetry.
Thus, in the case of a semiconductor crystal, for example, GaAs or
InP or ternary or quartenary compounds thereof, exhibiting space
group symmetry F43m, a polarization selective modulator may be
formed with optical waveguides extended on a (001) surface. In some
embodiments the optical waveguides may be either buried waveguides
or ridge waveguides, so that light propagates in a [110] or [110]
direction while external electric fields are applied parallel to
the [001] direction. FIG. 6 depicts a semiconductor (001) surface
on substrate 635 overlaid with buffer layer 630. Waveguides 640 can
be formed parallel to a [110] direction. Reciprocal beam splitters
or combiners 610 can separate incoming light from one waveguide 605
into two substantially equal parts, each entering one waveguides
640. A structure 610 can be used as a combiner and combine light
from waveguides 640 into a single outgoing waveguide 605. In some
embodiments, structures 610 may be formed by means of multimode
interference in a resonance box.
[0073] The index of refraction of each waveguide forming the
modulator, for example a light flux modulator, may be varied by the
application of an electric field F.sub.z as shown in FIG. 4 and
FIG. 5. In some embodiments, this may be accomplished by the
application of an electric field on either electrode 620 or
electrode 621 or both at the same time but in opposing directions
while electrodes 625 provide the electrical return path. The
waveguides are oriented parallel to [110] or [110] on a (001)
semiconductor crystal surface. The waveguide orientation in
relation to crystal axes can help prevent modulation of TM
polarized light while availing maximum modulation for TE polarized
light.
[0074] FIG. 7 illustrates an apparatus transmitting optically coded
radio frequency data and providing radio frequency local oscillator
for analog decoding received radio frequency waves, in accordance
with some embodiments of the present disclosure. The polarization
and spectral content of a light stream 700 are illustrated with
reference to the coordinate axes y and x as a function of
perpendicular distance from the y-x plane. The POLG sub-assembly
305 (FIG. 3) can prepare an initial state of the light stream,
characterized by carrier wavelength .lamda..sub.o, spectral
sidebands .lamda..sup.+1, .lamda..sup.-1, and orthogonal
polarizations indicated by arrows corresponding to the polarization
of the light stream, collectively represented by group 710, as also
represented at position (2) in 300(b) of FIG. 3. The frequency
difference between the central carrier .lamda..sub.0 and each of
the two sidebands is
.delta.f=c/(.lamda..sub.0+.lamda..sup.+1)=c/(.lamda..sub.0-.lamda..sup.-1-
) as is also the case as illustrated at FIG. 3.
[0075] Group 715 is a rendition of the spectral composition and
state of polarization of the light stream after application of
phase modulator 320 and light flux modulator 340. POLG phase
modulator 320 and light flux modulator 340 can be configured to be
highly selective to polarization in accordance with crystal
symmetry and waveguide orientation as represented in FIG. 4, FIG.
5, and FIG. 6. Optical filter 720 can remove wavelength bands
around .lamda..sup.-1. Carrier wavelength .lamda..sub.o and data
sub-band .lamda..sup.+1 in group 725 can be passed to polarizing
beamsplitter 730, which separates the light polarized along y,
shown as group 725, from light polarized along x, shown as group
726. Data bearing, p-polarized light, 725 can impinge on a
photodiode 730 where the electric fields pertaining to
.lamda..sub.o and .lamda..sup.+1 can mix to give rise to an
electrical signal oscillating at radio frequency .delta.f which can
be amplified by amplifier 735 and filtered by electrical filter
740. Thereafter, the electrical signal can be radiated in space as
radio frequency waves 750 by antenna radiator 745. Un-modulated,
s-polarized, light group 726 can impinge on photodiode 755 and
produces an electrical signal that can also oscillate at frequency
.delta.f and can be used as the electrical local oscillator in a
radio frequency front-end receiver, group 781. The electrical
signal from photodiode 755 can be amplified by the radio frequency
amplifier 760 and can enter a phase-tracking loop sub-assembly 780.
Moreover, the electrical signal from photodiode 755 can comprise a
phase detector 765, a voltage controlled oscillator 770, and a loop
filter 775. The front-end receiver can comprise an antenna 790
configured to efficiently couple to electromagnetic waves in a
radio frequency band containing .delta.f and its data sidebands.
The radio frequency signal received from antenna 790 can be
amplified by amplifier 789 and frequency mixed with local
oscillator signal 797 from photodiode 755 by radio frequency mixer
787. The mixed output from 787 can be filtered by radio frequency
filter 785 to retrieve base band data, BB.
[0076] FIGS. 8 and 9 illustrate providing phase and quadrature
modulation means coding optical and radio frequency transmissions,
in accordance with embodiments of the present disclosure. The POLG
sub-assembly 305 can prepare the initial state of the light stream,
characterized by carrier wavelength .lamda..sub.o, spectral
sideband .lamda..sup.-1 and arrows indicating the orthogonal
electric field direction or polarization of the light stream 877 at
position (1) in 800(a), corresponding to position (3) in 300(b) of
FIG. 3. Sideband .lamda..sup.+1 does not appear in 800(a) as may be
blocked by optical filtering means 330 shown in drawing 300(a). The
frequency difference between the central carrier and the
.lamda..sup.+1 sideband is substantially the modulation frequency
.delta.f=c/(.lamda..sub.0-.lamda..sup.-1) as is the case in
conjunction with FIG. 3. Polarization maintaining optical fiber 810
provides means to preserve the state of polarization and is
characterized by slow axis direction 815 and alignment key 820
shown in cross section representation 815. Polarization splitter
830 can separate the light stream into two orthogonal components
whose flux can be equalized by a variable optical attenuator 840.
An optical delay 835, commensurate with 1/4.delta.f, can be
introduced in one of the optical streams to adjust the phase
between the signal .lamda..sup.-1 and the reference light
.lamda..sub.0 to affect optical phase and quadrature coding.
([Zheng December 2014]).
[0077] Quadrature data can be encoded on each of the two
orthogonally polarized light streams by light flux modulators 845
and 850. As shown in FIG. 8, modulator 850 is rotated 90.degree.
relative to modulator 845 about the axis of light input, as
indicated by rotation arrow 847, to align the modulation axis of
data encoders with the polarization of the light stream.
Additionally, as shown at FIG. 5, one light stream is rotated
45.degree. clockwise while the other light stream is rotated
-45.degree. counter-clockwise, by polarization rotators 855 and 860
which may be 1/2.lamda..sup.-1 plates. The state of polarization of
each light stream is designated by P1 or P2 and the relative
phases, designated by .PHI.1=+.pi./4 and .PHI.2=-.pi./4, are
controlled by optical delay 835. .PHI.=(.PHI.1-.PHI.2) is the phase
difference between the two radio wave of frequency
.delta.f=c/(.lamda..sub.0+.lamda..sup.-1) in light streams P1 and
P2. At this point these radio waves exist only as different optical
frequencies in the optical domain; after photo-diodes 875 and 876
they will exist in the electrical radio frequency domain and one
will be retarded by 90.degree. relative to the other, or will be in
quadrature.
[0078] The state of polarization and data sidebands for light in
each orthogonally polarized stream 900 is depicted in FIG. 9 at
numbered positions corresponding to numbered positions along the
light path at 800(a) of FIG. 8: after emerging from the POLG 805,
position (1); after data coding, position (2); after polarization
rotators 855 and 860, position (3); after combining the two
orthogonally polarize light streams by combiner 865, position (4);
and after the two orthogonally polarized light streams are
recovered at the destination and re-separated as orthogonally
polarized lights, position (5).
[0079] The polarization combiner 865 can aggregate the two
spatially separated, orthogonally polarized, data bearing light
streams into one spatially coincident light stream whose state of
polarization and spectral content at position (4) is depicted in
drawing 900 of FIG. 9. The two aggregated light streams can carry
encoded radio frequencies from the optical light stream encoder
sub-assembly 898 to the analog in-phase and quadrature radio
frequency transmitter 899 in the optical domain over long
distances, signified by optical fiber loops 870.
[0080] In some embodiments, each of the two orthogonally polarized
light streams carries data and radio frequency carrier .delta.f.
The relative phases of the two radio frequency carrier waves in the
two light streams is controlled by optical delay 835 and if the
phase difference is .pi./2, then when the two beams are translated
to the electrical domain and combined, as with a Wilkinson power
combiner, the system constitutes an in-phase and quadrature radio
transmitter without using mixers, frequency synthesizers or
additional oscillators. Furthermore, because each constituent
wavelength of each orthogonally polarized light stream originates
from the same source and substantially follows the same channel
path, they are highly phase coherent, resulting in minimum phase
noise in the radio frequency carrier wave generated therefrom.
[0081] As shown at 800(b) of FIG. 8, the state of polarization of
the aggregate light stream can develop a certain amount of
ellipticity due to incidental optical fiber birefringence. This may
be corrected by polarization equalizer 873 before being
reconstituted into its two orthogonally polarized light stream
components P1 with relative phase .PHI.1=+.pi./4 and P2 with
relative phase .PHI.2=-.pi./4, by polarizing beam splitter 832.
Each beam P1 and P2 can then be incident respective photodiodes 875
and 876. Heterodyne mixing of parallel-polarized electric fields
can occur on each photodiode and can generate electrical radio
frequency signals that are amplified by amplifiers 883 and 884 and
can be carried, for example, to a Wilkinson power combiner 880, and
to an antenna radiator 890 over transmission lines represented by
broken lines 881 and 882. The radiated signal can include two radio
frequency carrier waves that are in relative phase delay of
90.degree. and each can carry quadrature encoded analog data. The
relative phase delay of the two carrier waves is depicted by
drawing sinusoidal forms 886 displaying relative 1/4 .lamda. phase
offset.
[0082] In another embodiment, the POLG apparatus, used in
conjunction with polarization selective modulators, can be used to
transmit in-phase and quadrature coded optical data accompanied by
a phase coherent light stream to be used as the optical local
oscillator at the coherent optical receiver. Referring to 800(a) of
FIG. 8, light from the optical light stream encoder sub-assembly
898 at position (4) is transmitted. The polarization components and
spectral content of the transmitted light stream at position (4) is
depicted at 900 at position (4) and at position (4) in drawing 1000
of FIG. 10, which depicts a simplified optical coherent detection
sub-assembly without a separate optical local oscillator.
[0083] Data 1010 (FIG. 10) entering the coherent detection
sub-assembly can comprise P1-polarized light bearing Ix-data and
orthogonally polarized P2 light bearing Qx-data. Polarizing
beamsplitter 1005 can separate the two polarizations into a Qx path
separate from the Ix path represented by directions 5(a) and 5(b).
Beamsplitter 1005 can perform a substantially similar function as
beamsplitter 832 in quadrature radio frequency transmitter 899 at
800(b) of FIG. 8. Light stream 5(a) bearing Ix data can be split
into two substantially equal channels by multimode interference
beamsplitter 1025 and each channel can be incident on one of a
matched photo-diode doublet 130. Light stream 5(b) bearing Qx data
can be split into two substantially equal channels by multimode
interference beamsplitter 1030 and each channel can be incident on
one of a photo-diode matched pair 130. Electrical amplifiers 135
can boost the signal before entering the digital signal processing
sub-assembly 1100.
[0084] In FIG. 10, there is no local optical local oscillator. The
accompanying central carrier wavelength .lamda..sub.0 is used
instead. The central carrier can be spectrally shifted from the
data band and can be substantially phase coherent with the data
light stream since both are derived from the same laser diode 310
in the POLG and follow substantially the same channel path.
Simplification at receiver digital signal processing end can be
balanced by slightly increased complexity in light stream
preparation at the analog front end in conjunction with analog
polarization ellipticity correction by polarization equalizer 873
in 800(b).
[0085] In some embodiments, a POLG apparatus, when used in
conjunction with polarization selective modulators, can demonstrate
how to use optical methods to transmit analog radio frequency data
in a format generally referred to as "frequency hopping spread
spectrum" relating to the transmitting and receiving of secure data
in wireless radio frequency communications.
[0086] Referring to the embodiment shown in FIG. 11, the phase
modulator 320 can be driven at sinusoidal frequencies that vary
cyclically between an upper and a lower limit in a predetermined,
truncated sequence, for example, a Fibonacci sequence. Sinusoidal
waveforms can be generated by frequency synthesizer 1110 as
determined by the programmed protocol of the logical frequency
sequencing unit 1105. A first consequence of driving the phase
modulator 320 in a frequency hopping mode is the spectral hopping
of the modulation sidebands .lamda..sup.+1 and .lamda..sup.-1 which
are spectrally separated by optical frequency difference
2.delta.f(t). The transmutation of frequency 2.delta.f(t) from the
optical to the electrical domain occurs when the two light streams,
having wavelengths .lamda..sup.+1 and .lamda..sup.-1, are
heterodyne mixed on a photo-diode. Each temporary radio frequency
band is used for transmission and reception during a finite dwell
time, as determined by the logical frequency sequencing unit 1105.
During the frequency dwell time each of the hopping radio
frequencies, 2.delta.f(t), may carry portions of a message. The
complete message may be obtained if the frequency sequencing
protocol is previously shared between sender and receiver.
[0087] The communication sequence in the embodiment depicted in
FIG. 11 may be summarized as follows. The state of polarization and
principal spectral sidebands of the light stream are prepared by
the POLG sub-assembly 305 as discussed in conjunction with FIG. 3.
Light emanating from the POLG comprises a central carrier
wavelength .lamda..degree. and orthogonally polarized, phase
modulation sidebands .lamda..sup.+1 and .lamda..sup.-1, as depicted
at position (2) at 300(b) of FIG. 3. Polarization maintaining fiber
810 guides light to polarization beamsplitter 830 where the central
wavelength .lamda..sup.o is diverted to other uses not relevant to
this specific exemplary embodiment. The linear polarization of
sidebands .lamda..sup.+1 and .lamda..sup.-1 transmitted by
polarizing beamsplitter 830 are rotated 45.degree. by polarization
rotator 855 and the orthogonally polarized components of the light
stream are separated by polarization beamsplitter 831 into two
light streams 1120 and 1125. Light in stream 1120 is orthogonally
polarized to light in stream 1125. Optical filter 720 removes
sideband X' from light stream 1120 while optical filter 721 removes
sideband .lamda..sup.-1 from light stream 1125. Remaining sideband
.lamda..sup.-1 in light stream 1120 is then encoded with data by
modulator 850 while remaining sideband .lamda..sup.+1 in light
stream 1125 is not modulated but its linear polarization is rotated
by 90.degree. to become parallel to the linear polarization in
light stream 1120.
[0088] Parallel polarized light streams 1120 and 1125 are then
merged into one light stream by light combiner 1115. Because two
different light paths are produced at polarizing beamsplitter 831,
the phase coherence condition between the two sidebands in the
original light stream 1130 is violated. The function of phase
retardation device 321 is to compensate for substantially slow
drifts in the phase coherence between the two sidebands, within an
integral multiple of 2.pi., by acting on the slow average drift
signal detected by photo-diode 875.
[0089] After the two parallel polarized light streams are combined
by combiner 1115, the resulting light stream contains the
un-modulated sideband .lamda..sup.+1 which is polarized parallel to
the co-propagating, data bearing sideband .lamda..sup.-1. Upon
heterodyne mixing on photo-diode 875, electrical amplification 735
and electrical filter 720, the resulting frequency hopping radio
frequency signal is transmitted by antenna radiator 745 as
represented by structure 1140. Coherence of the two optical
sidebands .lamda..sup.+1 and .lamda..sup.-1 that generate a steady,
non-fading radio frequency carrier wave, having frequency
2.delta.f(t), is maintained by detecting substantially slow average
photo-current drift by photo-diode 875. A slow current drift
indicates a walk-off the coherence state. The coherent state can be
closely maintained by adjusting a variable applied voltage,
F.sub.z, on phase retardation device 321 in accordance with
variations of the monitoring current from photo-diode 875 through
feed-back loop 1135. One example of a phase retardation device,
321, may be an optical waveguide composed of a zinc blende crystal,
said waveguide oriented on a (001) zinc blende crystal surface such
that light in the waveguide propagates substantially parallel to a
[110] direction and the magnitude of phase retardation is
proportional to the applied external field F.sub.z in accordance
with Eq. 1 or 2, for example, n.sub.2(F.sub.z).about.n.sub.o+1/2
n.sup.3r.sub.41F.sub.z.
[0090] The specific configurations, choice of materials and the
size and shape of various elements can be varied according to
particular design specifications or constraints requiring a system
or method constructed according to the principles of the present
disclosure. Such changes are intended to be embraced within the
scope of the present disclosure. The presently disclosed
embodiments, therefore, are considered in all respects to be
illustrative and not restrictive. The scope of the present
disclosure is indicated by the appended claims, rather than the
foregoing description, and all changes that come within the meaning
and range of equivalents thereof are intended to be embraced
therein.
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